extractAPI fails with unsafeNulls and Yexplicit-nulls

[bishabosha]

Compiler version

3.1.0

Minimized code

// -Yforce-sbt-phases -Yexplicit-nulls
import java.nio.file.FileSystems

def directorySeparator: String = {
  import scala.language.unsafeNulls
  FileSystems.getDefault().getSeparator()
}

Output

-- Error: foo.scala:5:39 -------------------------------------------------------
5 |  FileSystems.getDefault().getSeparator()
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |undefined: java.nio.file.FileSystems.getDefault().getSeparator # -1: TermRef(OrType(TypeRef(ThisType(TypeRef(NoPrefix,module class file)),class FileSystem),TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Null)),getSeparator) at sbt-api
1 error found

Expectation

extractAPI maybe ignores this?

[noti0na1]

I think this issue is similar to #13976: the imported language feature is not avaiable after typer (there is no error if we enable the unsafeNulls globally using -language:unsafeNulls).

When we transform FileSystems.getDefault().getSeparator in PostTyper, the type of FileSystems.getDefault() is nullable. We will get wrong type at .getSeparator because unsafeNulls is not visible at this phase.

We can always add a special attachment to the tree in typer if needed. I'm wondering if there is a better and general solution.

[olhotak]

See #14346 for a sketch of a general solution.